More admin clock time increments

This is quite a hack (need to modify Django code), but relatively simple and stable. It displays various times in whichever increments and columns you specify, rather than just Midnight, Noon, Now & 6am. You can use it throughout your admin and newforms-admin code.

This is a (slightly updated) copy of the ticket #1848 which wasn't included in trunk. http://code.djangoproject.com/ticket/1848